Controlled braking of a wheel set may be distributed over the various wheel brakes, or it may be organized to alternate over various groups of brakes, in which case half of the brakes are actuated while the other half of the brakes are released. When an aircraft is taxiing, i.e. running on the ground while not taking off or landing, and generally at low speeds of less than 70 km/h to 80 km/h, it may be advantageous to actuate a fraction only of the wheel brakes in order to improve passenger comfort and/or reduce wear on carbon brakes, thus giving rise to braking that is smoother than when all of the brakes are actuated together. Different groups of brakes are then actuated in alternation so as to avoid some of the brakes becoming overheated relative to others.
In any event, regardless of whether braking is performed on all of the brakes or on a fraction of them only by alternating between groups of brakes, it is always desirable to optimize braking accuracy as much as possible, in particular at low speeds and when small amounts only of deceleration are required.
The problem of controllability when braking at low speed and at low deceleration is particularly severe for large-capacity aircraft since the brakes are so powerful that it is often difficult to achieve an accurate and repeatable relationship between brake pedal position and aircraft deceleration, but the better the pilot manages to control braking, the greater passenger comfort.
About 20 years ago, proposals were made to organize a system for controlling disk brakes by servo-controlling braking torque as a function of aircraft speed, e.g. by using two different servo-controlled relationships, as illustrated in U.S. Pat. No. 4,043,607. That type of system is not very satisfactory when it comes to providing braking that is repeatable and accurate for given depression of the brake pedals.
More precisely, as described in EP-A-0 329 373, proposals have been made to enable at least one of the brakes to be disabled or inhibited selectively, while taking care to avoid disturbing the directional stability of the aircraft. The technique used for disabling the brakes is an on/off technique as a function of applied pressure, and disabling is applicable only at small or moderate depressions of the brake pedals. As a result, the transition from a fractional braking mode (in which only some of the brakes are actuated) to nominal braking mode (in which all of the brakes are actuated) necessarily disturbs continuity of braking (where discontinuity in the pressure applied to the brakes can affect passenger comfort, in particular) and/or disturbs the repeatability of braking as a function of pedal depression (the discontinuity in the perceived braking sensation can make it difficult for the pilot to get a "feel" for how hard the brake pedals need to be pressed).
Reference may also be made to EP-A-0 384 071 in which a system is proposed that uses the same technique of disabling a group of brakes (the word used is "inhibiting"), with disabling taking place as a function of measured wheel speed.
The state of the art is also illustrated by document U.S. Pat. No. 5,217,282 in the name of the present Applicant, in which electrohydraulic braking is organized to alternate between symmetrically disposed groups of wheel brakes by measuring the instantaneous temperature of each group of brakes and by selecting the group of brakes to be applied as a function of the temperature measurements performed. The braking apparatus is then controlled by a control unit which measures information that corresponds to brake pedal depression and which applies electrical control to servo-valves that are used for actuating the brakes. That technique is effective in minimizing brake heating, but it does not make it possible significantly to improve braking accuracy of large aircraft.
The state of the art is also illustrated by document EP-A-0 443 213 in which it is also desired to increase the lifetime of carbon brakes, by measuring aircraft speed and braking intensity, and by comparing the measured values with predetermined maximum values, so as to relax one or more brakes whenever the two measured values are less than said maximum values. The braking apparatus used is of the hydro-mechanical type, where the pressure of fluid delivered to the brakes is controlled by an associated braking manifold, said pressure (which is the same for all of the brakes) itself being modulated upstream as a function of the extent to which the brake pedals are depressed.
With such an apparatus, braking alternation is organized by providing an electro-valve associated with each brake in a group of brakes and arranged to suppress a braking order as from a certain threshold in the modulated pressure, with braking of the other group of brakes being ensured by the anti-skid valves associated with the brakes, which valves are not under pilot control.
Techniques of that type still suffer from the drawback of producing a certain amount of discontinuity in the applied pressure on going through the threshold, since the braking system switches from braking by means of one of two groups to braking by means of both groups using an overall pressure that doubles on passing through said threshold, thus having a direct effect on the braking sensation as perceived by the pilot, who is thus required to move the brake pedals accordingly in order to preserve passenger comfort. The main drawback that stems therefrom relates to the difficulty of keeping control after the brake pedals have been released. There is a risk that the pilot will be misled by the lack of precision between the extent to which the brake pedals are depressed and the sensation perceived during a preceding application of the brakes: if the pilot senses twice the deceleration for a given depression of the pedal, it is tempting to move the pedals repeatedly in order to keep control of braking, and this will have a direct effect on passenger comfort.
The technological background of the invention is also illustrated by the following documents: U.S. Pat. No. 4,365,847, U.S. Pat. No. 3,504,248, U.S. Pat. No. 5,116,108, and U.S. Pat. No. 5,116,109.